ArrayData: Allow defining array(s) on construction and make name optional in get_array

[sphuber]

Adds type hinting for aiida.orm.nodes.data.array.array.

Currently, the constructor does not allow to define any arrays to set
when constructing a new node, so one is forced to multi line code:

node = ArrayData()
node.set_array('a', np.array([1, 2]))
node.set_array('b', np.array([3, 4]))

This commit allows initialization upon construction simplifying the code
above to:

node = ArrayData({'a': np.array([1, 2]), 'b': np.array([3, 4])})

Note that it is also possible to pass a single array to the constructor,
in which case the array name is taken from the default_array_name
class attribute.

For backwards compatibility, it remains possible to construct an
ArrayData without any arrays.

The ArrayData was designed to be able to store multiple numpy arrays.
While useful, it forced users to be more verbose than necessary when
only storing a single array as an explicit array name is always required:

node = ArrayData()
node.set_array('some_key', numpy.array([]))
node.get_array('some_key')

The get_array method is updated to allow None for the name
argument as long as the node only stores a single array so that it can
return the correct array unambiguously. This simplifies typical user
code significantly:

node = ArrayData(numpy.array([]))
node.get_array()

[sphuber]

Any objections to this @mbercx @unkcpz ?

[unkcpz]

Thanks, looks all good.